1. Field of the Invention
The present invention relates to a process for producing ethylene-propylene block copolymers.
2. Description of the Related Art
In polymerization of olefins such as ethylene, propylene, etc., the performance characteristics of the polymerization catalyst used therefor have markedly advanced and the yield of polymer per unit weight of catalyst component has drastically improved in recent years. As a consequence, the amount of the transition metal catalyst component remaining in the resulting polymer has been decreased and the elimination of catalyst residue removal step has become possible.
Processes in use for the polymerization of these olefins include the slurry polymerization process conducted in inert hydrocarbon solvents, the bulk polymerization process conducted in liquified monomers such as liquified propylene, and the gas phase polymerization process conducted in a gas phase.
For producing block copolymers of ethylene and propylene, there is known a process which comprises the first step of polymerizing propylene or propylene containing a small amount of ethylene to form a propylene polymer or a propylene-ethylene copolymer and the second step of copolymerizing ethylene, or propylene containing a higher amount of ethylene than in the first step, with the first step polymer or copolymer.
In the above-mentioned process, however, usually the polymerization velocity in the second step is higher than in the first step and hence it is difficult to control the polymerization ratio in the second step. This tends to yield an increased amount of polymer of low stereospecificity or amorphous polymer in the second step. As a consequence, the quality of the product polymer is adversely affected. In particular, a gas-phase block polymerization process wherein the second step polymerization is conducted in the gas suffers from a number of disadvantages. There is a tendency for lumps of polymers to form in the polymerization vessel because polymer particles stick to one another. Similarly, the polymer tends to build-up on the interior surfaces of the polymerization vessel. Furthermore, the piping is apt to become fouled, and the polymer tends to stick to silos and hoppers. These disadvantages not only make it difficult to maintain normal operation but adversely affect the quality of products.